Metal tracks in integrated circuits are usually formed from aluminium by means of well known photolithography and etching techniques.
Usually a layer of aluminium is formed over the whole surface of a semiconductor substrate, and a pattern defined thereon in photoresist by exposing the resist through a mask and developing the resulting image.
After curing the developed image, aluminium which is no longer protected by the resist is removed, typically in a plasma etching process using a chlorine gas plasma.
After the pattern, defined by the photoresist image, has been etched the photoresist is removed from the remaining aluminium pattern in an asher. This is a plasma etching apparatus in which the resist is removed in an oxygen gas plasma while the substrate is heated.
Two kinds of asher may be used, a barrel asher such as the commercially available Des asher or a single substrate asher such as that manufactured by the company Matrix.
The barrel asher processes several substrates side-by-side together in a retaining boat, which is placed in a glass tube surrounded by a heating coil, to heat the environment containing the substrates to about 230.degree. to 240.degree. C.
In the single substrate system, the substrate is placed upon a heated chuck to heat the substrate directly.
A problem with both the above methods of resist removal is the occurrence of a staining appearance on the remaining aluminium after the removal of the photoresist in the asher.